When the radio link is configured in MIMO (multiple input multiple output) mode, then the downlink may transmit either one or two transport blocks in a single TTI (transmission timing interval), e.g., in the downlink HSDPA (high speed downlink packet access). This decision can be made dynamically, e.g., by a Node B scheduler. When one transport block is sent in the downlink, then one ACK/NACK indication is sent back to the Node B by the UE (user equipment). When two transport blocks are sent in the downlink, then two ACK/NACK indications (one for each transport block) can be sent in the uplink as provided within a hybrid automatic request process (HARQ).
There is a basic agreement in 3GPP (3rd Generation Partnership Project) that the single and dual ACK/NACK transmissions are sent in the same bit-field, meaning that when a single ACK/NACK is sent, then there is only one information bit to be coded, e.g., in the ACK/NACK field of the HS-DPCCH (high speed dedicated physical transport channel), but when two ACK/NACK transmissions are sent, then there are two information bits to be coded in the ACK/NACK field effectively halving the transmitted energy per information bit. Typically, the Node B knows in advance based on the downlink transmission whether to expect one or two ACK/NACK transmissions. Setting the power levels of the single/dual ACK/NACK in such a fashion should provide a high reliability of the dual ACK/NACK, but minimize energy used for the single ACK/NACK. In earlier solutions (e.g., see section 5.1.2.5A of 3GPP TS25.214, “Physical layer procedures (FDD)”, V5.11.0), an RNC (radio network controller), can signal, e.g., through the Node B to the UE, separate power offsets (levels) to be used for ACK and NACK transmissions (ΔACK and ΔNACK respectively).